1. Field of the Invention
This invention relates to air conditioning systems and more particularly to air conditioning systems having thermal storage capacity.
2. Prior Art
As automobiles and engines are made smaller in response to the increasing price and decreasing availability of fuel, the burden of the car air conditioner with respect to fuel economy and vehicular performance becomes more pronounced.
Air conditioner evaporators should be held at temperatures low enough to prevent reevaporation of condensed moisture and the discomfort in the conditioned space resulting therefrom, but high enough to prevent icing. In most automobile air conditioners these system requirements are met by running the compressor "continuously", and throttling the vapor exiting the evaporator to the extent necessary to prevent icing. This mode of operation has the desired effect, but results in substantial energy waste because under these conditions the compressor does additional work because of the throttling, and almost as much additional fuel is consumed in operating the air conditioner on a mild day as on a hot one. In those air conditioners which do not operate in this mode, but instead cycle the compressor during periods of light load, evaporator temperatures rise during periods of compressor shutdown. As explained above, this results in reevaporation of the moisture condensed on the evaporator and the discomfort associated with this humidity change. Moreover, even in this latter mode of operation the compressor will often be on in response to heavy cooling loads during periods when there is already an increased demand on the engine due, for example, to hill climbing or acceleration. During such periods the additional load imposed on the engine by the compressor seriously impairs vehicular performance, particularly in cars equipped with small engines.
It is thus apparent that an automobile air conditioner with thermal storage capability may be effectively employed to improve both fuel economy and vehicular performance since it can be cycled without noticeably affecting cooling. In addition, during periods of compressor shutdown, cooling from the thermal storage elements may be employed to maintain evaporator temperatures below levels associated with reevaporation of condensed moisture.
In the past, others have suggested the possibility of introducing some form of thermal storage capability into an air treatment system. Thus, U.S. Pat. No. 2,170,992 issued to Grady discloses an air conditioning system having thermal storage capability particularly adapted for use in an automobile air conditioning system. The system disclosed by Grady employs a single evaporator in which a heat-of-fusion thermal storage medium is disposed in surrounding relation with the evaporator refrigerant coil, and thus interposed between these coils and the air stream. While this arrangement does provide the desired thermal storage, cooling during periods of occasional use is delayed since the thermal storage medium must be cooled before effective air cooling will occur. This delay is particularly objectionable in automobiles and other applications where a substantially instantaneous stream of cool air is expected. A further disadvantage of the system disclosed by Grady is that the interposed medium constitutes an additional thermal resistance between the refrigerant and the air to be cooled. The result is that lower refrigerant temperatures are required to effect sufficient heat transfer from the air. This in turn results in reduced air conditioner efficiency and capacity.
U.S. Pat. No. 2,193,836 issued to Winther discloses an air conditioning system having thermal storage capability which employs two separate evaporators. One of the evaporators plays the conventional role while the other effects thermal storage for use during periods of light loads. Thus, Winther's system is advantageous insofar as it provides the expected substantially instantaneous cooling during occasional use. However, the cost and complexity of this system make it unattractive for use in motor vehicles and other applications.
U.S. Pat. No. 2,308,079 issued to Henney also discloses an air conditioning system having thermal storage capability which employs separate evaporators. Thus, like Winther's system, it is also unattractive for use in motor vehicles and other applications where cost reduction and system simplification are major objectives.
U.S. Pat. No. 3,293,409 issued to Snelling discloses an electric base board heat storage unit in which the heating element is disposed in thermal conducting relation with a heat storage element. Air to be heated is circulated through the base board unit across both elements such that during periods when the electric heating element is off, the circulated air will nevertheless contact the heat storage element thus heating the air and preventing the wide temperature variations normally present in thermostatically controlled electrical heating systems. However, this particular system is not well suited for applications such as motor vehicles where only occasional heating (or cooling) is required since, like Grady's system, it suffers from the inability to provide instantaneous heating (or cooling). This is so since the rate of heat transfer between the electric heating element and the thermal storage element is constant. Thus, during start up, the heating element must first heat the thermal storage element before maximum heat transfer to the circulating air may be effected.
U.S. Pat. No. 2,188,349 issued to Heideman discloses an air conditioning system having thermal storage capability which employs a single evaporator submerged in a freezable liquid. Thus, this system is quite similar to that disclosed by Grady and consequently possesses the same drawbacks. A similar system having similar drawbacks is disclosed in U.S. Pat. No. 2,752,763 issued to Shepard which discloses a beverage cooling apparatus having thermal storage capability.